As is well known, flat panel displays (FPDs) have become a mainstream of image display devices in recent years, the FPDs being typified by a liquid crystal display (LCD), a plasma display (PDP), a field emission display (FED), an organic light-emitting diode display (OLED), and the like. Progress is being made toward reducing the weight of those FPDs, and hence glass substrates used for the FPDs are also currently becoming thinner.
Further, there is a growing use of an organic light-emitting diode as a plane light source, such as a backlight of LCD and a light source for interior illumination, which emits only monochrome (for example, white) light, unlike a display that uses TFTs to blink light of three fine primary colors. Further, when an illumination device that uses the organic light-emitting diode includes a glass substrate having flexibility, a light-emitting surface is freely deformable. Therefore, from the viewpoint of ensuring sufficient flexibility, there is also promoted further thinning of the glass substrate (forming the glass substrate into a glass film) to be used for the illumination device.
In general, a method of cutting the glass substrate to be used for those FPDs and illumination devices involves a scribing step of forming a scribe having a predetermined depth in a front surface or a back surface of the glass substrate, followed by a breaking step of separating the glass substrate by applying a bending moment across the scribe line.
As an example of improvement for this type of glass substrate separating method, Patent Literature 1 discloses the following method. Specifically, an initial crack is formed at an end portion on a bottom surface of the glass substrate, and a heating unit for locally heating the glass substrate and a cooling unit for cooling the heated region scan the bottom surface of the glass substrate to form a scribe line extending from the initial crack. In a subsequent step, rollers that roll on the glass substrate sandwiched therebetween separate the glass substrate with the scribe line defined as a boundary.
Further, Patent Literature 2 discloses such a full-body cleaving method that thermal stress cleaving for a workpiece made of a brittle material (for example, glass substrate to be used for FPDs) is performed in separate steps that involve distribution of a thermal stress and propagation of a crack at a speed corresponding to a stress propagation rate as an upper limit, and a temperature distribution is formed by combining the heating through laser beam irradiation and the cooling through thermal conduction.